1. Field of the Invention
The field of the present invention is a molding apparatus for producing a synthetic resin molded product, and particularly, a molding apparatus for producing a molded product by utilizing an injection molding process, comprising a forming mold which includes a molding cavity, a gate communicating with the molding cavity, and an overflow chamber communicating with the molding cavity.
2. Description of the Prior Art
There is a conventionally known molding apparatus of such a type including, in order to form a hollow molded product, a gas blow-in mechanism for blowing gas into molten synthetic resin, which has been injected into the molding cavity, through the gate so as to form a void in the molten synthetic resin, which has been and a flow-out passage always open to the overflow chamber (see for example, Japanese Patent Application Laid-open No. 215516/90). Herein, the term "hollow molded product" means a shell-like molded product having a void over the entire region thereof and also a molded product having a void in a portion thereof.
With the above prior art apparatus, however, when the molten synthetic resin has been injected into the cavity, the resin tends happens to flow out through the flow-out passage into the overflow chamber. When such a situation occurs, the following problems are encountered: An injection pressure is insufficiently effected to the molten synthetic resin in the molding cavity, resulting in a deteriorated outer surface of the hollow molded product; and the amount of the resin in the cavity is insufficient, resulting in a hole generated in the body of the hollow molded product caused by the blown gas, and resulting in a non-uniform wall thickness.
In the injection molding process, after forming the molded product by injecting the molten synthetic resin into the molding cavity, a dwell pressure is required for inhibiting the shrinkage of the molded product. It is a conventional practice to provide such a dwell pressure by setting the injection pressure of the molten synthetic resin at a high level.
With the prior art apparatus, however, when the molded product is large in size, the injection pressure must be correspondingly increased. Therefore, to inhibit the generation of a flash, a mold clamping force must be increased, resulting in a problem of an increase in equipment cost.